Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into sectors. Information is written to and read from a disk by a transducer that is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different tracks. The disk is rotated by a spindle motor at high speed which allows the transducer to access different sectors on the disk.
A conventional disk drive, generally designated 10, is illustrated in FIG. 1. The disk drive comprises a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a bearing assembly 26. The actuator arm assembly 18 also contains a voice coil motor 28 which moves the transducer 20 relative to the disk 12. The spin motor 14, voice coil motor 28 and transducer 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device.
The disk drive 10 typically includes a plurality of disks 12 and, therefore, a plurality of corresponding actuator arm assemblies 18. However, it is also possible for the disk drive 10 to include a single disk 12 as shown in FIG. 1.
FIG. 2 is an air-bearing surface view of a transducer 20. As shown in FIG. 2, the transducer 20 includes functionally separate write and read heads 40, 42, respectively. More specifically, the transducer 20 typically includes a write element 200, a write gap 204, a first shield 208, a second shield 212, a read gap 216 and a magneto-resistive read element (or giant magneto-resistive read element) 220.
FIG. 3 is a simplified top view of a disk 12 illustrating a conventional embedded servo system (also known as sectored servo system). As illustrated in FIG. 3, the disk 12 includes a plurality of concentric tracks 44 for storing data on the surface of the disk. Although FIG. 3 only shows a relatively small number of tracks (i.e., 8), for ease of illustration, it should be appreciated that typically many thousands of tracks are included on the surface of a single disk 12.
Each track 44 is divided into a plurality of data sectors 46 and a plurality of servo sectors 48. The servo sectors 48 in each track are radially aligned with servo sectors 48 in the other tracks, thereby forming servo wedges 50 which extend radially across the disk 12. The servo sectors 48 are used to position the read head 42 and write head 40 associated with each disk 12 during operation of the disk drive 10.
In general, the transducer 20 has three modes of operation. First, the transducer 20 may read servo data located within servo sectors 48 using the read head 42. Second, the transducer 20 may read customer data located within data sectors 46 using the read head 42. Third, the transducer 20 may write customer data into data sectors 46 using the write head 40.
As is well understood by those skilled in the art, the first mode of operation of the transducer 20 (i.e., reading servo data) is used in conjunction with the second and third modes of operation (i.e., reading customer data and writing customer data, respectively). Specifically, the servo data must be read in connection with performing the operations of reading and writing customer data so that the read head 42 and the write head 40 are properly positioned over the disk surface (i.e., over the correct track 44 and data sector 46) when such operations are performed.
More specifically, when performing a read operation, servo data located in servo sectors 48 is read by read head 42 to properly position the read head 42 both radially and circumferentially over the disk 12. Customer data is then read from one or more data sectors 46 using the read head 42. While customer data is being read from the disk 12, the read head 42 is unable to make adjustments in its position over the disk 12. Accordingly, servo data located in servo sectors 48 is periodically read to make positional adjustments of the read head 42.
Similarly, when performing a write operation, servo data located in servo sectors 48 is read by read head 42 to properly position the write head 40 both radially and circumferentially over the disk 12. Customer data is then written to one or more data sectors 46 using the write head 40. While customer data is being written onto the disk 12, the read head is unable to make adjustments in the position of the write head 40 over the disk 12. Accordingly, servo data located in servo sectors 48 is periodically read by the read head 42 to make positional adjustments of the write head 40. It should be noted, however, that the write current to the write head 42 is turned off prior to the read head 42 reading servo data.
As is understood by those skilled in the art, the read head 42 and write head 40 are spaced apart from one another by a small distance in a downtrack direction (see FIG. 2). (In some cases, the read head 42 and write head 40 may also be spaced apart from one another by a small distance in a cross-track direction.) Because of this spacing, the read head 42 and write head 40 are generally unable to follow the same path over the disk 12. The amount by which the paths of the read and write heads 42, 40 differ (i.e., the skew) depends upon the radial position of the transducer 20 over the disk 12. Accordingly, the track over which the read head 42 is positioned will generally be different from the track over which the write head 40 is positioned. Thus, when performing a write operation, the position of the transducer 20 is adjusted after reading servo data from a servo sector 48 to properly position the write head 40 over the appropriate data track 44. This adjustment is known as “microjogging.”
Information is capable of being written onto or read from the surface of the disk 12 because the disk 12 is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. Writing is performed by delivering a write signal having a variable current to the write head 40 while the write head 40 is held close to the track 44 and data sector 46 to which such data is to be written. The write signal creates a variable magnetic field at a gap portion of the write head 40 that induces magnetic polarity transitions onto the surface of the disk 12 which constitute the data being stored.
Reading is performed by sensing the magnetic polarity transitions on the rotating track with the read head 42. As the disk 12 spins below the read head 42, the magnetic polarity transitions on the track present a varying magnetic field to the read head 42. The read head 42 converts the varying magnetic field into an analog read signal that is then delivered to a read channel (not shown) for appropriate processing. The read channel converts the analog read signal into a properly-timed digital signal that can be recognized by a host computer system (not shown).
More specifically, a read head 42 typically has an MR read element (or a GMR read element), which includes a strip of magneto-resistive material that is generally held between two magnetic shields. The resistance of the magneto-resistive material varies almost linearly with applied magnetic field. During a read operation, the MR strip is held near a desired track, with the varying magnetic field caused by the magnetic transitions on the track. A constant DC current (known as a bias current) is passed through the strip resulting in a variable voltage across the strip. By Ohm's law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host computer.
One problem with many conventional disk drives is that they suffer from a phenomenon known as write-induced instabilities. That is, when performing a write operation, the magnetic flux generated by the write head 40 to record information onto the disk surface has an affect on the read head 42 (after the write current is turned off) which prevents the read head 42 from properly reading servo data from the servo sectors 48. Specifically, during a write operation, after the write current to the write head 40 has been turned off and the read head 42 is being used to read servo data to properly position the write head 40, the read head 42 may not be able to properly read servo data from servo sectors 48.
More specifically, an instability is created in the domains of the read head 42 (e.g., in one or both of the shields, permanent magnet or MR/GMR element) due to the flux generated by the write current that is applied to the write head 40 while writing customer data onto the disk 12. This instability causes a large perturbation for a short period of time (e.g., 1-2 microseconds). Accordingly, the read signal generated when reading servo data from a servo sector 48, immediately after writing customer data to a data sector 46, is corrupted. Thus, the write head 40 may be positioned in an erroneous location over the surface of the disk 12 or the read head 42 must be permitted to re-read the servo data (by the disk spinning an entire revolution) before resuming the write operation. As will be appreciated, the performance of the drive, which is required to write information with high speed and high accuracy, can be significantly diminished by write-induced instability errors.
In order to ensure that disk drives 10 having transducers 20 that suffered from write-induced instabilities were not shipped to consumers, several tests were developed. Many of these tests require a drive to be manufactured before testing occurs. Accordingly, when a drive fails one of these tests, it is required to be discarded or reassembled (with new read head(s)), resulting in reduced manufacturing yield and manufacturing throughput, as well as, increased manufacturing costs.
For example, one of these tests is known as a WIPE test (write-induced position error test). In the WIPE test, many write operations are performed on each disk surface and every time servo data is determined to be corrupted, such errors are recorded on a head-by-head basis. The instability error rate for each head of a drive is then compared to a target instability error rate to determine whether a drive is within acceptable parameters to be sold to consumer. Again, if a drive fails the test, it is either discarded or disassembled, resulting in a loss of time and money for a disk drive manufacturer.
It would be advantageous to provide a method and apparatus for improving servo stability in MR heads while writing. More specifically, it would be advantageous to provide a method and apparatus for reducing instances of write-induced instabilities, so that manufacturing yields may be increased, manufacturing throughput can be increased and manufacturing costs can be decreased.